Cytokines represent a large number of secreted proteins that regulate cell growth and differentiation. These factors are especially important in regulating immune and inflammatory responses, and in regulating lymphoid development and differentiation. Not surprisingly, cytokines are critical in the pathogenesis of autoimmune diseases such as rheumatoid arthritis, SLE, IBD and psoriasis. Conversely, mutations that affect cytokines and cytokine signal pathways underlie a variety of primary immunodeficiencies. We discovered human Jak3, a kinase essential for signaling by cytokines that bind the common gamma chain, gc (IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21). We found that a mutation of Jak3 results in the primary immunodeficiency disorder SCID. After activation of receptor-associated Jaks, the next step in signal transduction is the activation of latent, cytosolic transcription factors that can also bind activated cytokine receptors, known as STATs. The function of STAT proteins in immune cells has been a focus of this lab for two decades. Work by us and other NIH scientists have revealed that mutations of STAT3 underlie the autosomal dominant form of hyperimmunoglobulin E syndrome (HIES). STAT3 has critical roles in immune cells and thus, hematopoietic stem cell transplantation (HSCT) might be a reasonable therapeutic strategy in this disease. However, STAT3 also has critical functions in nonhematopoietic cells and dissecting the protean roles of STAT3 is limited by the lethality associated with germline deletion of Stat3. Consequently, the potential efficacy of HSCT for HIES is difficult to predict. To begin to dissect the importance of STAT3 in hematopoietic and nonhematopoietic cells as it relates to HIES, we generated a mouse model of this disease. We found that these transgenic mice recapitulate multiple aspects of HIES, including elevated serum IgE and failure to generate Th17 cells. We found that these mice were susceptible to bacterial infection that was partially corrected by HSCT using wild-type bone marrow, emphasizing the role played by the epithelium in the pathophysiology of HIES. Many cytokines that activate STAT3 can also activate STAT1. This year we examined the relative contributions of STAT1 and STAT3 in signaling by two related cytokines, IL-6 and IL-27 and employed genetic models of cells lacking STAT1 or STAT3 and chromatin immunoprecipitation-sequencing (ChIP-seq) to better understand the function of these transcription factors. Pertinent to this project, we used T cells from patients at the NIH Clinical Center with gain-of-function STAT1 mutations. These patients are susceptible to fungal infections and have impaired Th17 differentiation, like HIES patients. We found that STAT3 is responsible for the overall transcriptional output driven by both cytokines, whereas STAT1 is the principal driver of specificity. STAT1 cannot compensate in the absence of STAT3 and, in fact, much of STAT1 binding to chromatin is STAT3-dependent. However, STAT1 shapes the specific cytokine signature superimposed upon STAT3's action. In our previous work, we have identified BACH2 as a critical transcription for T and B cell homeostasis. The BACH2 locus also has a very striking super-enhancer architecture. This year we have begun analyzing cells from patients with potential mutations of this interesting transcription factor.